1. Field of the Invention
The present invention relates to a solid-state imaging device and imaging apparatus that detect the optical black level of an image signal in order to obtain an image having favorable contrast.
2. Description of the Related Art
Typically a dark current in a solid-state imaging device changes depending on the temperature and the accumulation time. Therefore, a signal output value in the optically-black state, i.e., in the no-exposure state, changes depending on the temperature and the accumulation time, which often affects an image. For example, when a certain fixed signal level is used as the basis of A/D conversion, the digital value of an optical black level will change depending on the temperature. Accordingly, a black part in an image obtained after image processing will float as a whitish area or sink as an intense-black area depending on the temperature, which results in unstable contrast of the image.
In order to avoid such a problem, in many solid-state imaging devices, light-shielded pixels (hereinafter, referred to also as optical black (OB) pixels) that are shielded from light are provided around aperture pixels. By reading out these OB pixels, an optical black level can be output on a frame-by-frame basis. Thus, even when the temperature changes, the optimum reference level for A/D conversion can be estimated.
FIG. 13 is a diagram showing one example of a related-art solid-state imaging device. FIGS. 14 and 15 show an example of signal outputting from the related-art solid-state imaging device.
As shown in FIG. 13, this solid-state imaging device includes an aperture pixel part 201 having plural aperture pixels 211 for capturing an image, an OB pixel part 202 having plural OB pixels 212 provided around the aperture pixel part 201, a V selection circuit 203, and a read-out circuit 204. From the aperture pixels 211 of the aperture pixel part 201 and the OB pixels 212 of the OB pixel part 202, OB pixel rows, aperture pixel rows, and OB pixel rows are sequentially selected by the V selection circuit 203 in accordance with horizontal synchronizing signals, and the pixel values of the selected pixels are read out by the read-out circuit 204.
As shown in FIG. 14, outputting of an image of one frame is triggered by a vertical synchronizing signal (XVS) and is completed until the next vertical synchronizing signal. In the period between the vertical synchronizing signals, signals of each one row are output in accordance with horizontal synchronizing signals (XHS) serving as triggers of the signal outputting.
For example, FIG. 15 shows an example of signal outputting when the number m1 of former OB pixel rows is three and the number m2 of latter OB pixel rows is two. As shown in FIGS. 13 to 15, the output values of the OB pixels 212 read out in an initial period of one frame are sampled, and the sampling result is applied to the black level of the aperture pixels 211 in this frame. This can cancel temperature changes within one frame even when the temperature sharply changes.
However, as pointed out also in Japanese Patent Laid-open No. Hei 10-107245 (Patent Document 1), it is known that when a light-shielding film is provided above pixels, it is difficult to strictly equalize the dark current amount of the aperture pixels to that of the OB pixels because of difference in the surface level therebetween and so on. Therefore, the dark current amount differs between the aperture pixels and the OB pixels even when there is no change in the use temperature within one frame. In the worst case, this difference possibly affects image contrast. Furthermore, even when the difference in the dark current amount is small, problems often occur if the accumulation time is extended for capturing of a dark image for example.
There is another method in which photodiodes are not provided in OB pixels in order to suppress the dark current in the OB pixels. However, in this method, the dark current in the OB pixels is extremely smaller than that in the aperture pixels. As a result, an error in the black level occurs depending on the temperature and the accumulation time.